Photometric modeling of Asteroid 5535 Annefrank from Stardust observations

Abstract In this paper the Stardust disk-integrated phase curve at phase 47.2–134.6° of the Asteroid 5535 Annefrank, combined with groundbased observations (at phase 2.3–18.3°), are fit with Hapke’s photometric model. We confirm Newburn et al.’s (Newburn, R.L. et al. [2003]. J. Geophys. Res. 108 (E11), 5117. doi: 10.1029/2003JE002106 ) observation that Annefrank exhibits a steep phase curve. This manifests itself in an unusually high fit surface roughness parameter of 49°. The single particle scattering albedo is 0.62, also high for an S-asteroid, while the fit phase function is more forward scattering than the typical S-asteroid being nearly isotropic with an asymmetry parameter of −0.09. The fit opposition surge width ( h  = 0.015) is typical of S-asteroids. However these fits assume a spherical shape to the asteroid. Li et al. (Li, J., A’Hearn, M.F., McFadden, L.A. [2004]. Icarus, 415–431) have shown that this assumption may lead to significant errors particularly at high phase angles leading to higher modeled single particle scattering albedos, macroscopic roughnesses and more forward scattering phase functions than actually exhibited. Our results confirm this finding—fitting only the data below 90° phase yields lower particle albedos (0.41) and roughnesses (20°) and more backscattering particles (−0.19) than the fit including the high phase angle data. Overall Annefrank appears to be on the bright side but otherwise is typical for an S-type asteroid suggesting that it may be a recent collisional fragment with a relatively immature surface which has had relatively little time to be weathered.

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